To control the quality of integrated circuits, semiconductor wafers or other substrates are inspected for defects by optically scanning the surface with a variety of inspection apparatus. Commonly, wafers have a mirror-like surface which is reflective of light, but many common optical inspection tools rely upon dark field scattering of light from particles and defects for inspection purposes. In these optical inspection tools illumination of the surface inspected produces two types of optical signals, the desired scattered light signals and an undesired specular (reflected) light signal which is commonly suppressed. The scattered light signals are derived from both defect scattering and from diffuse surface scattering known as “haze”. The specularly reflected signal is the incident beam minus the components that are lost by scattering and absorption at the surface. In conventional tools, an exit aperture in the light collection optics or a beam dump has been used to rid the scattered light collection system of the undesired reflected beam signal.
At various stages of wafer processing, thin metallic and/or dielectric films, such as oxide or nitride layers, are deposited over the surface. Typically, such films demonstrate a certain amount of “transparency” to the wavelengths of light used to inspect the surface (an inspection beam). Thus, the inspection beam can penetrate the top layer(s) of the wafer to reach underlying (less transmissive) layers of the film. Such thin films have a thickness of between 10 angstroms and several tens of microns. It is important to know, for example, oxide thickness over the surface for several reasons. Uniformity of an oxide layer may be desired so that semiconductor devices employing the oxide layers will have uniform properties. The oxide layers are insulative and the electrical performance of semiconductor devices, such as transistors, rely upon an insulative layer of a particular thickness. Additionally and importantly, the deposition of thin layers can degrade defect (e.g., particle) measurement information obtained by analysis of the scattered light.
FIG. 1 is a simplified schematic illustration of an illumination beam 101 being directed onto a defect 102 (in this case a particle) present on a wafer 103 surface. The defect scatters the electric field at the surface creating scattered light signal S as well as a reflected light signal R in the specular direction. Such scattering is well known in the art and can be used to identify and quantify defects on a wafer surface. A complicating factor in the depicted illustration is the presence of one or more partially transmissive layers 104 on the surface of the wafer 103. Such partially transmissive layers have the effect of transmitting certain wavelengths of light through the layer. This is particularly troublesome when the layer is transmissive to the wavelength(s) of light of the illumination beam 101. Such partially transmissive layers can be comprised of many different materials, but particularly common examples include silicon oxides and silicon nitrides. As can be seen in FIG. 1, a portion 101a of the illumination beam 101 can penetrate into the at least partially transmissive layer 104 and be at least partly reflected by an underlying surface 105. This partly reflected portion 101b is also scattered by the defect 102 thereby contributing to the scattered light signal S. This can cause a significant variance to the expected scattered light signal. Thus, the scattered light that is typically used to characterize the defect 102 (for example to determine defect size) is now so altered that it exhibits significant deviations from an optical signal obtained from a surface having no partially transmissive layers.
Thus, thin layers of oxide or nitride (or other partially transmissive materials, including but not limited, to certain low-K dielectric materials) can significantly alter the radiation fields (light beams) and alter the expected scattering profile for a defect. In one example case, using a substrate having formed thereon a partially transmissive film that has an optical thickness of one-quarter of the wavelength of illuminating light, the strength of the radiation field at or near the surface is substantially reduced resulting in little light scattering for micron-sized defects lying on the surface. This can result in such defects being missed. Thus, knowledge of film thickness would be critical to compensating for this condition or avoiding measurements at such thicknesses.
Currently, tools exist that can measure film layer thickness. However, such tools are high precision tools that can only measure a few points on each wafer to determine thickness (commonly no more than 20-25 such measurements are made per wafer). Additionally and importantly, each measurement takes a long time to achieve. Thus, for such tools it is unrealistic to measure more than a few points on each wafer. As a result such tools provide accurate, but limited amounts of information and have no real capacity to characterize an entire surface. These tools are not capable of scanning an entire wafer and determining the thickness of a partially transmissive layer across the entire surface. Moreover, such tools cannot make thickness measurements while these tools are simultaneously making defect inspection measurements of the same surface.
Additionally, such tools cannot provide “wafer maps” of reflectivity behavior of an inspected surface. For example, current tools cannot provide high-density reflectivity measurements across an entire silicon wafer. Also, current tools cannot provide a reflectivity measurement map of an entire wafer having at least one layer of partially transmissive material formed thereon.
Additionally, for a variety of opaque films, it can be desirable to generate reflectivity maps of an entire wafer surface. This is particularly true for metal films, such as aluminum or tungsten, where knowledge of the reflectivity of the metallic layer, as well as surface scattering, can yield substantial information about the quality and uniformity of the film deposition. Current methodologies are not capable of scanning a surface and generating such maps. Additionally, such reflectivity maps of a surface can be used to enhance detection and yield a more accurate defect size distribution count, since defect scattering is strongly influenced by the surface optical parameters, such as reflectivity.
Additionally, current tools and methodologies do not have a way of rapidly scanning an entire wafer and using the wafer's reflectivity characteristics to determine critical dimension (CD) for the elements formed on the wafer's surface. Nor do current tools and methodologies have a way of rapidly scanning an entire wafer and using the wafer reflectivity characteristics to determine the alignment accuracy of a fabrication overlay during processing. Thus, there is a need for tools and methodologies capable of measuring surface reflectivity characteristics by continuously scanning a surface. Additionally, there is a need for tools and methodologies for measuring thin film thickness and characterizing defects in the same process. Also, there is a need for tools and methodologies capable of continuously scanning a surface to determine CD and determine the correct alignment of overlays constructed during fabrication.